On Mon, Jul 13, 2015 at 7:25 PM, Richard Smith <rich...@metafoo.co.uk> wrote:
> On Mon, Jul 13, 2015 at 6:02 PM, Adrian Prantl <apra...@apple.com> wrote: > >> >> On Jul 13, 2015, at 5:47 PM, Richard Smith <rich...@metafoo.co.uk> wrote: >> >> On Mon, Jul 13, 2015 at 3:06 PM, Adrian Prantl <apra...@apple.com> wrote: >> >>> > On Jul 13, 2015, at 2:00 PM, Eric Christopher <echri...@gmail.com> >>> wrote: >>> > >>> > Hi Adrian, >>> > >>> > Finally getting around to looking at some of this and I think it's >>> going in slightly the wrong direction. In general I think begin -able- to >>> put modules in object files to simplify wrapping, use, etc is a good thing. >>> I think being required to do so is somewhat problematic. >>> > >>> >>> Let me start with that the current infrastructure already allows >>> selecting whether you want wrapped modules or not by passing the >>> appropriate PCHContainerOperations object to CompilerInstance. Clang >>> currently unconditionally uses an object file wrapper, all of >>> clang-tools-extra doesn’t. We could easily control the behavior of clang >>> based on a (new) command line option. >>> >>> But.. on a platform with a shared module cache you always have to assume >>> that a module once built will eventually be used by a client that wants to >>> read the debug info. Think llvm-dsymutil — it does not know and does not >>> want to know how to build clang modules, but does want to read all the >>> debug info from a clang module. >>> >>> > Imagine, for example, you have a giant distributed build system... >>> > >>> > You'd want to create a pile of modules (that may reference/include/etc >>> other modules) that aren't don't or may not have debug information as part >>> of them (because you might want to build without it or have the debug info >>> alongside it as a separate compilation). Waiting on the full build of the >>> module including debug is going to adversely affect your overall build time >>> and so shouldn't be necessary - especially if you want to be able to have >>> information separate ultimately. >>> > >>> > Make sense? >>> >>> Not sure if you would be saving much by having the debug info >>> separately, from what I’ve measured so far the debug info for a module >>> makes up less than 10% of the total size. Admittedly, build-time-wise going >>> through the backend to emit the object file is a lot more expensive than >>> just dumping the raw PCH. [1] >>> >>> Yeah, I think wanting to be able to control the behavior is reasonable, >>> we just need to be careful what the implications for consumers are. If we >>> add a, e.g., an “-fraw-modules” [2] or switch to clang to turn off the >>> object file wrapping, I’d strongly suggest that we add the value of this >>> switch to the module hash (or add a an optional “-g” to the module file >>> name after the hash or something like that) to avoid ugly race conditions >>> between debug info and non-debug-info builds of the same module. This way >>> we’d have essentially two separate module caches, with and without debug >>> info. >>> >> >> That's fine, I think (we don't use a module cache at all in our build >> system; it doesn't really make much sense for a distributed build) and most >> command-line flag changes already have this effect. >> >> >> Great! >> >> >> >>> would that work for you? >>> -- adrian >>> >>> [1] If you want to be serious about building the module debug info in >>> parallel to the rest of the build, you could even have a clang-based tool >>> import the just-built raw clang module and emit the debug info without >>> having to parse the headers again :-) >>> >> >> That is what we intend to do :) (Assuming this turns out to actually be >> faster than re-parsing; faulting in the entire contents of a module has >> much worse locality than parsing.) >> >> [2] -fraw-modules, -fmodule-format-raw, -fmodule-debug-info, ...? >>> I would imagine that the driver enables module debug info when >>> "-gmodules” is present and by default on Darwin. >> >> >> That seems reasonable to me. For the frontend flag, I think a flag to >> turn this on or to select the module format makes more sense than a flag to >> switch to the raw format. >> >> >> Okay then let’s narrow this down. Other possibilities in that direction >> include (sorted from subjectively best to worst) >> >> -fmodule-format=obj >> -fmodule-debug-info >> -ffat-modules >> -fmodule-container >> -fmodule-container-object >> > > It's a -cc1 flag, so it doesn't really matter much. If this will > eventually govern whether we put code for inline functions into the module, > then I think we should avoid names like -fmodule-debug-info. Other than > that, I don't really have a preference. > What you're picturing there is essentially a flag that would indicate if we should build all module-related-object-things into the module, or not? That seems like a useful broad flag (with an eventual corresponding compiler mode where we pass another flag and explicitly pass just the module and say "build a separate object with all the module-related-object-things - for use in a non-implicit-cache build) (Hmm, we're going to have a weird middle ground in here - where the IR for the inline functions needs to go in the module itself (as an available_externally definition for use in non-LTO compilations of dependent object files) and then the build-separate-module-related-object-things would turn those into (weak?) definitions, compile them (& the debug info) into a separate object file, to be linked in at the end) Should this just be keyed/defaulted off implicit/explicit modules, or orthogonal to that choice? > [One other thing... I think we may have made a mistake by putting the >> reader and writer code behind the same interface: it forces tools that want >> to read the module format to link against all of LLVM IR, code generation, >> and so on, when all they really need is something like libObject.] >> >> >> We can always split it into two implementations of the interface or two >> interfaces, that’s not a very big deal. My assumption was that every tool >> that wants to read the clang module format also wants to create modules >> (because module cache... but as you noted that’s a Darwin-centric view) and >> more low-level tools like llvm-bcanalyzer could be piped through >> llvm-objdump. >> >> -- adrian >> > > > _______________________________________________ > cfe-commits mailing list > cfe-commits@cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits > >
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